1. Field of the Invention
The present invention is directed to a method for the presentation of a magnetic resonance image.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for, among other things, acquiring images of an inside of a body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance apparatus also has a radio-frequency system that radiates radio-frequency signals into the examination subject for triggering magnetic resonance signals and that picks up the generated magnetic resonance signal, on the basis of which magnetic resonance images are produced.
A good homogeneity of the basic magnetic field is a critical factor for the quality of the magnetic resonance images in magnetic resonance technology. Field inhomogeneities of the basic magnetic field within an imaging volume of the apparatus thereby cause geometrical distortions of the magnetic resonance image that are proportional to the field inhomogeneities. The same is true of non-linearities of the gradient fields.
A distortion-free and location-true imaging is important in many applications, for example in the employment of magnetic resonance images for planning radiotherapy of a tumor or for the preparation for or implementation of a surgical intervention.
Passive and active shim devices are known as measures for improving the basic magnetic field homogeneity. In the case of a passive shim device, a number of iron plates are attended in a suitable arrangement in the examination space of the magnetic resonance apparatus. The basic magnetic field within the imaging volume is measured before the attachment of the iron plates and a computer program determines the suitable number and arrangement of the iron plates therefrom. In an active shim device, shim coils are utilized that homogenize the basic magnetic field by suitable drive with direct currents are utilized. The shim coils usually are designed such that they essentially compensate a specific inhomogeneous field component corresponding to a coefficient of a spherical function representation of the basic magnetic field within the imaging volume.
Further, for example, German OS 198 29 850, corresponding to U.S. Pat. No. 6,252,401, discloses a method for the correction of distortions in magnetic resonance images as a consequence of inhomogeneities of the basic magnetic field and/or non-linearities of gradient fields. An image dataset belonging to the magnetic resonance image is suitably corrected with this method. However, limits are placed on the accuracy and non-ambiguity of the correction of distortions in direct dependence on the degree of the local inhomogeneity and/or non-linearity.
In some magnetic resonance apparatus, moreover, no further slices to be acquired can or should be planned on the basis of an image that has been distortion-corrected with the aforementioned method, since the slices fixed in the distortion-corrected image would not coincide with the actual conditions in the examination subject.
An object of the invention is to provide an improved method for the presentation of a magnetic resonance image with which, among other things, distortions contained in the magnetic resonance image can be taken into consideration in a simple way in further image-dependent planning.
The above object is achieved in accordance with the principles of the present invention in a method for displaying a magnetic resonance image wherein an identification is made, prior to the display of the magnetic resonance image for diagnostic purposes, of any regions of the image which contain distortions, and the magnetic resonance image is displayed with that region or those regions containing distortions being designated.
In accordance with the inventive method, a viewer, for example a diagnosing physician, can determine how greatly the displayed anatomy differs from xe2x80x9crealxe2x80x9d anatomy from the suitably designated magnetic resonance image at first glance and can appropriately take this into consideration in, for example planning a therapy or operation. Further, the viewer can plan further slices to be imaged in the designated magnetic resonance image and can appropriately take the distortions into consideration at the same time based on the enhanced informational content of the designated: magnetic resonance image.
In an embodiment, a distortion correction matrix wherein the distortions are encoded is superimposed on the magnetic resonance image for the designation. The distortion correction matrix indicates at what locations and how significantly the magnetic resonance image, or a region to be imaged must be corrected as a consequence of the inhomogeneities of the basic magnetic field and non-linearities of gradient fields that prevail in the region to be imaged during the image exposure. The distortion correction matrix is derived, for example, from a distortion correction method with which the magnetic resonance image is distortion-corrected. On the basis of the distortion correction matrix, a degree of the distortion is encoded, for example, chromatically and/or in terms of intensity, for example with a color scale from white to red. In combination with the distortion correction method employed, further, the distortion correction matrix can contain information about the extent of remaining residual distortions in the distortion-corrected magnetic resonance image as a result of inaccuracies and ambiguities of the distortion correction method. In one embodiment of the method, only the aforementioned residual distortions are chromatically encoded and/or encoded in terms of intensity. Locations of the magnetic resonance image for which an unambiguous distortion correction can be implemented are not designated.